I'm trying to diagnose a noise coming from my drivetrain. It's a knocking sound/creaking sound. I've put the bike on a stand but it doesn't do it. It also doesn't do it when i'm standing on it. Only when I'm sitting. Where do I even start? I'm obviously not a bike tech, should I just take it in? I have tried doing an extensive cleaning and re-oiling everything but it hasn't helped.